CVTs have a belt made up of a plurality of stacked metal rings for transmitting power. The stacked metal rings include a plurality of metal rings having slightly different circumferential lengths, which are successively stacked on outer circumferential sides of inner adjacent metal rings.
Generally, each of the metal rings, which have different circumferential lengths, is fabricated by cutting a cylindrical drum of maraging steel into a preform having a prescribed width, heat-treating the preform according to a solution heat treatment, an aging treatment, a nitriding treatment, etc., and correcting the circumferential length of the preform by elongating the preform in order to increase the circumferential length to a prescribed dimension (see, for example, Japanese Patent No. 3986995).
In some cases, a flat plate having a prescribed thickness is used instead of the cylindrical drum. Such a flat plate is curved until end faces thereof abut against each other, so that the flat plate is changed into a cylindrical shape. Then, the end faces, which are held in abutment against each other, are joined together. Thereafter, various heat treatments referred to above are performed.
For performing such heat treatments, it is the general practice to feed a plurality of metal rings held by a feed jig into a heat treatment furnace in which the metal rings and the feed jig are heated together. For example, the present applicant has proposed feed jigs of this type, as disclosed in Japanese Laid-Open Patent Publication No. 2002-161314 and Japanese Patent No. 4219186.
There are also known feed jigs, as disclosed in Japanese Laid-Open Patent Publication No. 2007-191788 and Japanese Laid-Open Patent Publication No. 2008-240085.
As shown in FIG. 3 of Japanese Laid-Open Patent Publication No. 2007-191788, the feed jig disclosed therein includes a base plate and six solid holder shafts vertically mounted on the base plate. Each of the solid holder shafts has a plurality of ring seats, which project horizontally from a side wall thereof. As shown in FIG. 4 of this publication, a metal ring is held by the feed jig, while the side wall of the metal ring is engaged between ring seats of the six solid holder shafts.
The feed jig disclosed in Japanese Laid-Open Patent Publication No. 2008-240085 also holds a metal ring between ring seats that project from side walls of six holder shafts, which extend parallel to each other (see FIGS. 6 through 8 of Japanese Laid-Open Patent Publication No. 2008-240085).
As described above, both of the feed jigs disclosed in Japanese Laid-Open Patent Publication No. 2007-191788 and Japanese Laid-Open Patent Publication No. 2008-240085 have ring seats that project from side walls of the holder shafts in the form of round rods in directions perpendicular to the direction in which the holder shafts extend. Tapered slanted surfaces are interposed between vertices of the ring seats and the side walls of the holder shafts. A metal ring is inserted between slanted surfaces of lower ring seats and slanted surfaces of upper ring seats (see FIG. 4 of Japanese Laid-Open Patent Publication No. 2007-191788).